The present invention relates to a vapor phase growth method and apparatus, and more particularly to a plasma chemical vapor deposition (CVD) method and apparatus.
In the art to which this invention pertains, the plasma chemical vapor deposition method has been widely practiced for forming a desired film on a substrate such as a semiconductor substrate. In this method, plasma is generated between oppositely arranged elecrodes in a reaction chamber of a reactor, and a deposition is formed on the substrate by the chemical phase reaction of reactive gases introduced into the furnace tube.
One plasma CVD apparatus used to carry out the method of vapor phase growth mentioned above is schematically illustrated in cross-section in FIG. 1. This apparatus is of the so-called parallel plate type or condenser coupled type. If silicon-nitride film is to be attached or grown on the surface of a semiconductor substrate, for example, air is drawn out of a reaction chamber 1 made of quartz or stainless steel (in which case it must be shielded in the conventional manner) through an exhaust pipe 2 to maintain a vacuum pressure on the order of 1 Torr within the reaction chamber 1. The pressure will be indicated by a vacuum gauge 8. Monosilane (SiH.sub.4) gas and ammonia (NH.sub.3) gas are introduced through an inlet 3 into the reaction chamber via control taps or valves 10a, 10b and flow meters 9a, 9b. Radio frequency power generated by a generator 13 is applied, through a conventional matching box 14, between upper and lower electrodes 4 and 5 which are oppositely arranged to cause discharge therebetween. Matching box 14 is used to match the output impedance of the generator 13 to the input impedance of the apparatus within the reaction chamber 1. The lower part of the upper electrode 4 is a porous plate having openings. Lower electrode 5 is a two layer structure base made of aluminum covered stainless steel. The lower electrode 5 is heated, by a conventional heater 6, to 300.degree. to 400.degree. C. A standard thermocouple 11 is placed under the heater 6, and, through a suitable feedback controller 12, is connected to a conventional power source (not shown) for the heater 6. Semiconductor substrates 7 are placed parallel to each other on the lower electrode 5. Reactive gases are introduced into the reaction chamber 1 and are caused to jet onto the substrates 7, heated to 300.degree. to 400.degree. C., through openings in the porous plate of the upper electrode 4, the gases being excited by the discharge energy to effect vapor phase reaction. As a result, Si.sub.3 N.sub.4 film attaches or grows on the semiconductor substrates 7.
The advantage of the above method of vapor phase growth over known ordinary chemical vapor deposition methods is that the desired film is grown on the semiconductor substrate at temperatures in the range of 300.degree. to 400.degree. C. compared to a temperature on the order of 850.degree. C. employed in conventional chemical vapor deposition methods. In the manufacture of integrated circuits, it is desired to carry out the process of manufacture after the metallization process at temperatures not exceeding 450.degree. C. because metals having low melting points are used in the interconnection of integrated circuit elements. In the method explained above, the temperature within the reaction chamber never exceeds 450.degree. C., so that this method satisfies the temperature requirements for the manufacture of integrated circuits.
The condition of the deposition grown depends on whether the frequency used is high or low. If a low frequency is selected, a film of high quality is produced but the deposition rate is low. In contrast, if a high frequency is selected, the deposition rate is high, but the quality of the film is not satisfactory. This is a major problem encountered in carrying out the conventional vapor phase growth method in which a radio frequency power source is utilized.
As an example of the problems mentioned above, the prior art method was carried out using the apparatus shown in FIG. 1 under a variety of conditions.
For the purpose of growing a film of Si.sub.3 N.sub.4 as described above, a power source frequency of 400 KHz was selected. The ratio of gases introduced was set so that NH.sub.3 /SiH.sub.4 =2. A radio frequency power in range of 10 to 30 W was used while maintaining the vacuum pressure within the reaction chamber 1 at 1 Torr. The resulting deposition rate was 150 .ANG./minute. Although the deposition rate was low, a high quality Si.sub.3 N.sub.4 film having a small number of pinholes was grown.
Under the above conditions, the power source frequency was changed to 13.56 MHz. A high deposition rate of 500 .ANG./minute was achieved, but there were many pinholes, hence poor film quality.
The experimental results obtained by carrying out the plasma chemical vapor deposition method by applying a single radio frequency power source, including the two examples described above, are shown graphically in FIG. 2 which shows the relationship between the frequency and the deposition rate and pinhole density. The applied power was 30 W, or the power density per unit area of the electrode was 0.3 W/cm.sup.2. In FIG. 2, the abscissa represents the power frequency and the ordinates the deposition rate at left and the pinhole density at right. In FIG. 2, curve A illustrates the relationship between the power frequency and the deposition rate and curve B illustrates the relationship between the power frequency and the pinhole density.
This phenomenon is due to the high rate of growth of silicon ions and nitrogen ions or silicon radicals and nitrogen radicals which bring about not only the growth of Si.sub.3 N.sub.4 on the surface of semiconductor substrate but also the falling down and deposition of Si.sub.3 N.sub.4 produced through reaction in the vapor phase.
Further, if high frequency power is applied, the thermocouple 11 beneath the lower electrode 5 functions as an antenna, and thus produces noises that affect the feedback controller 12 which is liable to cause unwanted temperature rise. On the other hand, plasma generated by low frequency power is unstable. Therefore, whether high frequency or low frequency power is used, there are both advantages and disadvantages.